1. Field of the Invention
The present invention relates to an exhaust discharge control device for an internal combustion engine.
2. Description of the Related Art
It is assumed that the ratio of the entire air amount to the entire amounts of fuel and a reducing agent supplied into an exhaust passage, a combustion chamber and an intake passage upstream of a certain position within the exhaust passage is referred to as an air-fuel ratio of an exhaust gas flowing at that position. Here, there is a known exhaust discharge control device for an internal combustion engine that allows burning of a lean air-fuel mixture gas, in which an NO.sub.x absorbent that absorbs NO.sub.x if the air-fuel ratio of the inflowing exhaust gas is lean and discharges the absorbed NO.sub.x if the oxygen concentration of the inflowing exhaust gas is low, is disposed in the exhaust passage of the internal combustion engine such that the air-fuel ratio of the exhaust gas flowing into the NO.sub.x absorbent is made rich or a stoichiometric air fuel ratio temporarily to discharge and reduce the absorbed NO.sub.x from the NO.sub.x absorbent (see Japanese Patent Publication No. 2600492).
If the oxygen concentration of the exhaust gas flowing into the NO.sub.x absorbent is reduced, NO.sub.x or SO.sub.x is discharged and removed. Based on this, it is considered that as the oxygen concentration of the exhaust gas flowing into the NO.sub.x absorbent becomes lower, NO.sub.x or SO.sub.x is purified more excellently, and if oxygen is hardly contained in the exhaust gas flowing into the NO.sub.x absorbent, NO.sub.x or SO.sub.x is can be purified further excellently.